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�u " r►uAM ✓tom—/�A Working to Restore Nature Previous Environmental Work In May and June 1988, RESNA (formerly Applied GeoSystems) collected nine soil samples from test pits excavated near the dispenser islands and observed removal of three 12,000- gallon tanks at the site (Applied GeoSystems, July 8, 1988) The tanks were replaced with two gasoline tanks, which are presently at the site Soil samples collected from the test pits and beneath the tanks contained gasoline hydrocarbons The excavation was terminated due to the proximity to the road and existing structures Hydrocarbon-containing soil was left in place Nineteen soil borings have been drilled at or near the site Thirteen of these borings were converted into groundwater monitoring wells, four were converted into vapor extraction wells, one into a pumping test well, and one soil boring was backfilled The initial five monitoring wells were installed in January and February 1990 and have been predominantly dry since October 1990 Three additional wells and the four vapor extraction wells were installed in March 1992 The final five monitoring wells were installed offsite in August 1992 and delineate dissolved hydrocarbons in groundwater (RESNA, December 16, 1992) In March 1992, a vapor extraction test was performed at the site (RESNA, June 29, 1992), and in August 1992, a groundwater pumping test was performed onsite (RESNA, May 4, 1993) REGIONAL GEOLOGY AND HYDROLOGY Unocal Service Station 6981 is in the Central Valley geomorphic province of California, which is a northwestward-trending, asymmetric structural trough that has been filled with 6 to 10 miles of sediment ranging in age from Jurassic to Holocene (Page, 1986) The topographic surface is nearly flat with a slight westward slope Based on an extrapolation of a geological map prepared by Marchand and Atwater (1979), sediments beneath the site consist of Quaternary alluvial deposits of the Modesto Formation, which were deposited by the Mokelumne River The nearest surface stream is the Calaveras River which trends east and west approximately 1,800 feet southeast of the site PRESENT INVESTIGATION The work was performed according to RESNA's Work Plan 38032-14W (RESNA, August 18) and Site Safety Plan 38032-93S (RESNA, January 1993) Details of the drilling operations, soil sampling, and laboratory analyses are presented below Our field methods are described in an attachment to this report Copies of the Chain of Custody Records and laboratory reports for soil samples are attached . 1214dcam/38032-14 2